a) Field of the Invention
The present invention concerns polymer electrolyte batteries, for example, polymer electrolyte generators having a potassium salt enabling to stabilize the performances and service life of the battery. More specifically, the invention relates to rechargeable lithium generators containing a potassium salt which is distributed in the cathode, in the polymer electrolyte, or both of these at the same time. In particular, the present invention is directed to rechargeable electrochemical generators in which the potassium ions introduced in the form of additives in the cathodes as well as in the polymer electrolyte define an in situ treatment which lasts the entire service life of the generator so as to improve performances during cycling, for example, in terms of energy and power. The present invention also concerns potassium base additives distributed in at least one, and even two, of the components of a rechargeable lithium electrochemical generator, preferably the polymer electrolyte and the composite cathode bound to a polymer, in which the effect is to stabilize the energy and power performances during cycling.
The invention also describes preferred means for introducing potassium into the generator by either one of its components and describes how the potassium is distributed in more than one component so as to optimize the operation of the electrodes during cycling. The additive has the beneficial effect of maintaining the morphology of the lithium anode during cycling and to optimize the physical properties of the cathode during cycling.
b) Description of Prior Art
The life of a battery is dependent on many factors including the reversibility of the electrochemical processes at both electrodes. The addition of alkali earth or transition metals to the active material of the cathodes of lithium batteries is known and is used generally to stabilize or optimize the insertion cathodes (FR 2,616,013; U.S. Pat. No. 5,013,620; U.S. Pat. No. 5,114,809; FR 2,573,250). The additives used are generally intended to stabilize the insertion structures and sometimes to optimize the number of sites available in the host structure (WO 91/02383; U.S. Pat. No. 4,668,594). In some cases, the additives are also intended to increase the electronic conductivity of the insertion materials (U.S. Pat. No. 4,965,151; JP 89/15317; JP 89/67063; U.S. Pat. No. 5,114,811; U.S. Pat. No. 5,147,737). In most of the cases known, the addition metals are integrated in the host structure and are present at relatively high rates which vary between 1% and 50% with respect to the main transition metal. These additives are generally immobilized in the host insertion structure and are not diffused in the other components of the generator, for example, the electrolyte and the anode. In the case where the addition metals would be soluble in the electrolyte, they would be reduced with metallic lithium and could not remain in equilibrium in the generator. Moreover, in Applicant""s view, no example of additive which is present in more than one component of a lithium generator has been described up to now.
As a matter of fact, a few of these metals are chemically compatible with a lithium anode and are capable of coexisting with the lithium salts which are in solution in the electrolyte of the generator. Potassium, with magnesium is one of the only metals which are not reduced (thermodynamically and kinetically) by lithium in aprotic media and therefore constitutes a unique material to carry out the present invention.
The utilization of polymer electrolytes with mixed alkali cation has been mentioned during conductivity measurements (ACFAS 1993) and for the constitution of vehicular conduction electrolytes (cf. U.S. Pat. No. 5,350,646). None of these cases mention an equilibrium between the mixed cations of the electrolyte and the materials of the electrode or a beneficial effect on the cycling of the generator or on the lithium anode.
It is an object of the present invention to provide for a beneficial effect noted on the stabilization of the material of the cathode V2O5 which is presently used in the technology of polymer electrolyte batteries and simultaneously to provide an improvement to the reversibility of the dissolution-redeposition of the lithium anode and its morphology in order to improve the performances and service life of the battery.
It is therefore an object of the invention to provide improvements by the introduction of a potassium salt, such as KTFSI (potassium trifluoromethanesulfonyl imide) in the polymer separator and/or in the polymer which constitutes the composite positive or cathode.
It is another object of the invention to introduce potassium ions in the polymer, which are subsequently present in the electrodes by electrochemical means.
It is also an object of the invention to provide for the manufacture of a battery characterized by an improved cycling with respect to energy and power obtained by in situ addition of a stabilizing agent, such as potassium, without necessarily utilizing a chemical means.
It is another object of the invention to permit the introduction of a stabilizing agent, such as in the polymer electrolyte separator and/or in the cathode, so that it is uniformly distributed in the entire battery from the interface Li to the collector of the positive.
It is also an object of the invention to produce depolarizing effects during recharge (decrease of voltage of more than 20 mV in the case of dendrite formation, contact of the anode with the cathode) following the morphological development of lithium (modification of a plane surface of lithium into a rugged surface), which enables the battery to cycle again in a reversible manner for many tens of cycles without any appearance of dendrites.
In order to achieve these objects and to overcome the disadvantages of the prior art, the invention proposes a rechargeable lithium battery including at least one lithium anode, one lithium ion reducible cathode bound with a first polymer, and a polymer electrolyte comprising a second polymer, and a lithium salt in solution in the second polymer. The lithium generator according to the invention is characterized in that it contains potassium ions, which are distributed in at least one among the cathode and the polymer electrolyte, the concentration of lithium and potassium under equilibrium in the second polymer expressed as O/(Li+K), being between about 8/1 and 40/1, with a Li/K molar ratio between about 0.2 and 15. The potassium ions are selected so as to stabilize the energy and power performances of the generator during cycling.
In general, the potassium ions are introduced by means of potassium salts. The potassium salt may be distributed in the cathode, in the electrolyte, or both in the polymer electrolyte and the cathode.
Moreover, the first and second polymers may be identical or different depending on circumstances, as this will appear to one skilled in the art.
Among the potassium salts that may be used according to the invention, KN(RFSO2)2  are KN(RFSO2)2, KN(FSO2)RSO2  KN(FSO2)RFSO2, KBF4, KPF6, KRFSO3  and KRFSO3, where F  each RF is independently a perhalogenoalkyl or a perhalogenoaryl and where R is an organic radical which could be the same or different, KBF4 and the like may be mentioned , it being understood that the list is not limited to these compounds. The potassium ions may be incorporated in the cathode in partially or completely reduced form.
Preferably, the cathode includes at least one compound selected from oxides, sulfides or chalcogenides of transition metals, for example, vanadium pentoxide.
The compound which constitutes the cathode may be selected among those represented by formula
[xe2x80x94Rxe2x80x94Sx]n
in which R is a di- or tri-radical. Examples of di-radicals include for example sulfur, alkylene groups containing 2 to 10 carbon atoms, oxyalkylene groups containing 4 to 12 carbon atoms, and 1 to 4 oxygen atoms, cyclic radicals such as substituted or unsubstituted phenylene, thiadiazodi-yl and oxadiazodi-yl. An example of tri-radical includes a derivative of 1,3,5 triazine on which three sulfur atoms are substituted, n is the degree of polymerization, which is comprised between 2 and 100,000, preferably between 10 and 10,000, and X is xe2x89xa72, the potassium then being present in the cathode in the form of R-S-K where R is such as defined above. A mixture of polymers comprising at least one electronically conductive polymer and a compound of the type [xe2x80x94Rxe2x80x94Sx]n may also constitute the cathode. It may also be selected among those represented by formula
MXz
in which M is a transition metal, X represents a chalcogen or a halogen, and z varies between 1 and 3, potassium then being present in the cathode in the form of KX, where X is such as defined above.
The invention also concerns a lithium ion reducible cathode, for rechargeable lithium generator, bound to a polymer, potassium ions being distributed in the cathode in such quantity that the concentration of lithium and potassium in a polymer electrolyte of a generator made of cathode when the generator has reached equilibrium, expressed as O/(Li+K) varies between about 8/1 and 40/1 while the molar ratio Li/K is between about 0.2 and 5.
The invention also concerns a polymer electrolyte for a rechargeable lithium battery, potassium ions being distributed in the electrolyte in such quantity that the concentration of lithium and potassium in the polymer electrolyte of a generator including the latter when the generator has reached equilibrium, expressed in O/(Li+K) varies between about 8/1 and 40/1 while the molar ratio Li/K is between about 0.1 and 5.
The amount of potassium salt, such as KTFSI with respect to the lithium salt currently used in a polymer electrolyte generator, varies to a large extent and is preferably within a K/Li ratio lower than 5, preferably between 0.2 and 1.
When the cathode contains V2O5, the maximum concentrations of Kxcex1V2O5, are preferably those where xcex1xe2x89xa60.06 or still where K/V is lower than or equal to 0.03.